Linear Photodiode Array Detector Research Articles

1.3 μm single photon emission from InAs/GaAs quantum dots

Single-photon emitters are crucial for the applications in quantum communication, random number generation and quantum information processing. Self-assembled InAs/GaAs quantum dots (QDs) have demonstrated to have singlephoton emission with high extraction efficiency, single-photon purity, and photon indistinguishability. Thus they are considered as the promising deterministic single-photon emitters. To extend the emission wavelength of InAs/GaAs QDs to telecom band, several methods have been developed, such as the strain engineered metamorphic quantum dots, the use of strain reducing layers and the strain-coupled bilayer of QDs. In fact, it is reported on single-photon emissions based on InAs/InP QDs with an emission wavelength of 1.55μm, but it is difficult to combine such QDs with a high-quality distributed Bragg reflector (DBR) cavity because the refractive index difference between InP and InGaAsP is too small to obtain a DBR cavity with high quality factor. Here we investigate 1.3μm single-photon emissions based on selfassembled strain-coupled bilayer of InAs QDs embedded in micropillar cavities. The studied InAs/GaAs self-assembled QDs are grown by molecular beam epitaxy on a semi-insulating (100) GaAs substrate through strain-coupled bilayer of InAs QDs, where the active QDs are formed on the seed QDs capped with an InGaAs layer, and two-layer QDs are vertically coupled with each other. In such a structure the emission wavelength of QDs can be extended to 1.3μm. The QDs with a low density of about 6×10<sup>8</sup> cm<sup>-2</sup> are embedded inside a planar 1-<sup>λ</sup> GaAs microcavity sandwiched between 20 and 8 pairs of Al<sup>0.9</sup>Ga<sup>0.1</sup>As/GaAs as the bottom and top mirror of a DBR planar cavity, respectively. Then the QD samples are etched into 3μm diameter micropillar by photolithography and dry etching. The measured quality factor of studied pillar cavity has a typical value of approximately 300. Photoluminescence (PL) spectra of QDs at a temperature of 5 K are examined by using a micro-photoluminescence setup equipped with a 300 mm monochromator and an InGaAs linear photodiode array detector. A diode laser with a continuous wave or a pulsed excitation repetition rate of 80 MHz and an excitation wavelength of 640 nm is used to excite QDs through an near-infrared objective (<sup>NA</sup> 0.5), and the PL emission is collected by the same objective. The time-resolved PL of the QDs is obtained by a time-correlated single photon counting. The second-order correlation function is checked by a Hanbury-Brown and Twiss setup through using ID 230 infrared single-photon detectors.<br/>In summary, we find that the 1.3μm QD exciton lifetime at 5 K is measured to be approximately 1 ns, which has the same value as the 920 nm QD exciton lifetime. The second-order correlation function is measured to be 0.015, showing a good characteristic of 1.3μm single photon emission. To measure the coherence time, i.e., to perform highresolution linewidth measurements, of the QDs emitted at the wavelength of 920 and 1300 nm, we insert a Michelson interferometer in front of the spectrometer. The obtained coherence time for 1.3μm QDs is 22 ps, corresponding to a linewidth of approximately 30μeV. Whereas, the coherence time is 216 ps for 920 nm QDs, corresponding to a linewidth of approximately 3μeV. Furthermore, both emission spectral lineshapes are different. The former is of Gaussian-like type, while the latter is of Lorentzian type.

Linear array of photodiodes to track a human speaker for video recording

Communication and collaboration using stored digital media has garnered more interest by many areas of business, government and education in recent years. This is due primarily to improvements in the quality of cameras and speed of computers. An advantage of digital media is that it can serve as an effective alternative when physical interaction is not possible. Video recordings that allow for viewers to discern a presenter's facial features, lips and hand motions are more effective than videos that do not. To attain this, one must maintain a video capture in which the speaker occupies a significant portion of the captured pixels. However, camera operators are costly, and often do an imperfect job of tracking presenters in unrehearsed situations. This creates motivation for a robust, automated system that directs a video camera to follow a presenter as he or she walks anywhere in the front of a lecture hall or large conference room. Such a system is presented.The system consists of a commercial, off-the-shelf pan/tilt/zoom (PTZ) color video camera, a necklace of infrared LEDs and a linear photodiode array detector. Electronic output from the photodiode array is processed to generate the location of the LED necklace, which is worn by a human speaker. The computer controls the video camera movements to record video of the speaker. The speaker's vertical position and depth are assumed to remain relatively constant– the video camera is sent only panning (horizontal) movement commands. The LED necklace is flashed at 70Hz at a 50% duty cycle to provide noise-filtering capability. The benefit to using a photodiode array versus a standard video camera is its higher frame rate (4kHz vs. 60Hz). The higher frame rate allows for the filtering of infrared noise such as sunlight and indoor lighting–a capability absent from other tracking technologies. The system has been tested in a large lecture hall and is shown to be effective.

Analytical Characteristics of a Continuum-Source Tungsten Coil Atomic Absorption Spectrometer

A continuum-source tungsten coil electrothermal atomic absorption spectrometer has been assembled, evaluated, and employed in four different applications. The instrument consists of a xenon arc lamp light source, a tungsten coil atomizer, a Czerny-Turner high resolution monochromator, and a linear photodiode array detector. This instrument provides simultaneous multi-element analyses across a 4 nm spectral window with a resolution of 0.024 nm. Such a device might be useful in many different types of analyses. To demonstrate this broad appeal, four very different applications have been evaluated. First of all, the temperature of the gas phase was measured during the atomization cycle of the tungsten coil, using tin as a thermometric element. Secondly, a summation approach for two absorption lines for aluminum falling within the same spectral window (305.5-309.5 nm) was evaluated. This approach improves the sensitivity without requiring any additional preconcentration steps. The third application describes a background subtraction technique, as it is applied to the analysis of an oil emulsion sample. Finally, interference effects caused by Na on the atomization of Pb were studied. The simultaneous measurements of Pb and Na suggests that negative interference arises at least partially from competition between Pb and Na atoms for H2 in the gas phase.

Lowering the UV absorbance detection limit and increasing the sensitivity of capillary electrophoresis using a dual linear photodiode array detector and signal averaging

We have recently described a new spatial signal-averaging approach to improve the UV absorbance detection limits for capillary electrophoresis (CE) using a single photodiode array (PDA) detector. (Culbertson, C. T.; Jorgenson, J. W. Anal Chem 1998 70, 2629: Culbertson, C. T. PhD Dissertation, University of North Carolina, Chapel Hill, 1996). At low analyte concentrations, however, source flicker noise interfered with the signal-averaging process. This noise was due to inadequate compensation of the spatial and temporal lamp fluctuations that occurred over the course of a run in this single-beam instrument. To better compensate for the source lamp fluctuations and drift, a double-beam detector has been constructed using a dual PDA to continuously monitor the output of the source. The addition of the reference channel significantly reduces the source flicker noise. To demonstrate the capabilities of this new detector, separations of four nucleic acids at six different concentrations were performed. The signal-averaged electropherograms generated using the double-beam instrument are compared with electropherograms obtained from individual diodes in the array, a single-beam detector, and a commercial single-point detector. The detection limits for the double-beam instrument were 35(±3) times better than that obtained using an individual diode in the array, ca. 4 times better than that obtained using a single-beam detector, and 6.3(±0.4) times better than that obtained using a commercial single-point detector. The linearity of the new detector is found to be equal to or better than that of the commercial single-point detector, and a separation of analytes at mixed concentrations corresponding to a two order of magnitude dynamic range is shown. ©1999 John Wiley & Sons, Inc. J Micro Sep 11: 652–662, 1999

Lowering the UV absorbance detection limit and increasing the sensitivity of capillary electrophoresis using a dual linear photodiode array detector and signal averaging

Lowering the UV absorbance detection limit and increasing the sensitivity of capillary electrophoresis using a dual linear photodiode array detector and signal averaging

A continuum source vs. line source on the way toward absolute graphite furnace atomic absorption spectrometry

The main achievements in the development of the methods of absolute analysis in atomic absorption spectrometry during 40 years are based on the electrothermal atomization of samples and measurement of absorption coefficient in the centre of resonance lines with line-spectra sources as it was proposed by Walsh in 1955. Three main problems with this procedure are the following: (1) a non-homogeneous distribution of analyte vapours in the vertical direction of the graphite tube and its potential redistribution in the presence of matrix; and (2) much higher variations of sensitivity than expected due to the age and individual characteristics of line sources and operating conditions and (3) the deviation from a proportional relation between the absorbance and atomic vapour concentration. To eliminate the first problem, it has been suggested to use a linear photodiode array detector instead of a photomultiplier and to summate absorbances over the height of spectral slit of monochromator. To eliminate the second and third problems, it has been proposed to use a continuum source instead of line sources and to measure a total (wavelength integrated) absorption of resonance line with a high-resolution echelle spectrometer. The advantages of this last proposal are not so obvious as might be expected from some recent publications. From the analysis of different approaches to linearization and stabilization of calibration curves, it was concluded that the most simple and efficient way consists in implementation of the boosted-output hollow-cathode lamps in commercial instruments.

Direct Comparison of Near-Infrared Absorbance Spectroscopy with Raman Scattering Spectroscopy for the Quantitative Analysis of Xylene Isomer Mixtures

In order to overcome instrument problems associated with moving parts, such as optical misalignment and/or mechanical breakdown, we have developed two solid-state (no moving parts) spectrometers suitable for many industrial process monitoring applications. The first instrument utilizes near-infrared absorbance spectroscopy with a 1024-element platinum-silicide linear photodiode array detector, and the other employs Raman scattering spectroscopy with a 1024 × 1024 element charge-coupled device (CCD) detector. In order to demonstrate the utility of solid-state instrumentation for industrial process monitoring analysis, both instruments were used for the simultaneous quantitative analysis of individual components of xylene isomer mixtures. The xylene isomer mixture samples prepared for this study contained approximately 75–86, 0.6–5, and 0.1–14% w/v ortho-, meta-, and para-xylene, respectively, to reflect compositions of xylene raw materials used by specialty chemical manufacturers. Each spectroscopic system provides a means for fast (seconds), nondestructive data acquisition with no sample preparation. With the use of the chemometric data treatment of partial least-squares (PLS) regression, the absolute accuracies at 95% confidence for each isomer were found to be ±0.05, ±0.12, and ±0.09% w/v with near-infrared spectroscopy and ±0.08, ±0.04, and ±0.07% w/v with Raman spectroscopy for ortho-, meta-, and para-xylene, respectively.

High-Resolution, Time-Resolved Spectra of Indium and Aluminum Atoms, Fluorides, Chlorides, and Oxides in a Graphite Tube Furnace

High-resolution (4.4 pm), time-resolved (every 0.2 s during the atomization step) spectra of indium and aluminum atoms and molecules (InF, InCl, AlF, AlCl) were obtained in a graphite tube furnace with the use of a xenon arc lamp for excitation and a linear photodiode array detector. The spectra of indium fluoride, aluminum fluoride, and aluminum chloride were shown to be composed of several vibrational bands making up the (0, 0) sequence, while the indium chloride spectrum included vibrational progressions as well. For indium fuoride and aluminum fluoride, the diatomic molecule appeared temporally earlier than atoms of the metal. Spectra of reagents used to produce indium fluoride and aluminum fluoride were similar to previous spectra in the literature that were assigned to In2O and Al2O, respectively. The spectral overlap of the absorption band for aluminum chloride (261.44 nm) with a lead absorption line (261.42 nm) is discussed with respect to its implications for background correction for graphite furnace atomic absorption spectrometry.

Extended calibration ranges for continuum source atomic absorption spectrometry with array detection

Computer modeling has been used to construct calibration curves and relative concentration error plots for continuum source atomic absorption spectrometry with array detection and graphite furnace atomization. Model results are compared with experimental results obtained with a linear photodiode array detector. The model uses a Lorentzian absorption profile convoluted with a rectangular entrance slit (25, 50, 100, 200, or 500 μm wide) and detected with an array of pixels (each 25 μm wide) using a high resolution spectrometer. Transient furnace signals are modeled as triangular functions with a half-width of 2 s whose height and area are linearly dependent on concentration. With detector read noise limiting (characteristic of a photodiode array detector), the best signal-to-noise ratios have been obtained with a 500 μm entrance slit width and wavelength integrated absorbance (i.e. integration of absorbance over the whole absorption profile). The shapes of the modeled calibration curves agree well with those theoretically predicted and those obtained experimentally. Useful calibration ranges approaching six orders of magnitude of concentration have been achieved using a single calibration curve and integrating over a spectral region equivalent to 60 times the half width of the absorption profile (about 0.16 nm for Cd at 228.8 nm). When concentration is normalized by the intrinsic mass, all elements give the same curve shapes with the inflection point, from a slope of 1.0 to 0.5 (on a logarithmic scale), determined by the stray light. A hyperbolic function has been developed which accurately fits the modeled and experimental data. With photon shot noise limiting (characteristic of a charge coupled device), the signal-to-noise ratio is much less dependent on the entrance slit width. With a 25 μm entrance slit width, wavelength selected absorbances (i.e. absorbances computed for selected pixels or wavelengths) have been used to construct three calibration curves covering six orders of magnitude of concentration.

Sensitivities and detection limits for graphite furnace atomic absorption spectrometry using a continuum source and linear photodiode array detection

Analytical figures of merit have been examined for a series of elements (As, Cd, Co, Cu, Fe, Mn, Ni, Pb, Se, Sn, and Zn) using graphite furnace atomic absorption spectrometry (GFAAS) with a continuum source and a linear photodiode array detector. Intrinsic masses, m i were calculated for continuum source AAS based on wavelength and time integrated absorbance in units of pm-s. A simple triangular model, based on experimental values for the hollow cathode lamp linewidths and calculated widths for the absorption profiles, was used to compute m i values from tabulated characteristic mass values for conventional AAS. With the exception of As and Se, the m i values for continuum source and conventional AAS agreed within a factor of two. Continuum source detection limits obtained were similar to those reported for line source GFAAS. Two standard reference materials were analyzed and values for the elements studied were found to be in good agreement with the certified values.

Multicomponent kinetic determination of lanthanides with stopped-flow, diode array spectrophotometry and the extended Kalman filter

The application of the extended Kalman filter to multicomponent kinetic data is described. The method is based on obtaining data at multiple wavelengths over time using a linear photodiode array detector. The extended Kalman filter is used to process the data obtained. It is shown that accurate results can be obtained even if the estimated value of the rate constant is not completely accurate or reproducible. No pH, ionic strength, or temperature controls were used in testing the chemical system. A system of three lanthanides reacting with 4-(2-pyridylazo)resorcinol (PAR) was used. Accurate estimates of concentrations were obtained even though the relative rate constants for the reactions of La, Pr, and Nd with PAR were 1:1.7:1.9, and a high degree of spectral overlap is present. 23 refs., 4 figs., 4 tabs.

High current pulsing of a xenon arc lamp for electrothermal atomic absorption spectrometry using a linear photodiode array

A series of 300 and 500 W xenon arc lamps, normally operated at 20 and 35 A, respectively, have been pulsed as high as 300 A to achieve higher intensities in combination with a linear photodiode array detector. Initial tests without pulsing showed that the 500 W lamps are generally more intense, but the 300 W lamps were more intense at 200 nm. With 300 A pulses, both lamps showed a factor of 500 increase in the pulse intensity over the simmer intensity. With a 0.5 ms pulse and a 3.75% duty cycle, pulsing at 300 A provided a factor of 18 increase in the integrated intensity over normal d.c. operation. The increase in integrated intensity can result in a comparable improvement in detection limits since the instrument is detector noise limited. Both the 300 and 500 W lamps exhibited failure after the equivalent of 200 atomizations at 200 A. With 100 A pulses, the 300 W lamp was still operating after the equivalent of 800 atomizations. At both pulse levels, the decrease in intensity with time was accelerated as compared with d.c. operation. It was concluded that an improved lamp design is necessary to make pulsed operation economically attractive.

The effect of spectral bandpass on signal-to-noise ratios for continuum source atomic absorption spectrometry with a linear photodiode array detector

Equations describing the optical throughput of a monochromator are used as a basis for modeling the dependency of the signal-to-noise ratio on the monochromator parameters and the limiting noise. In-depth evaluation is made for the detector noise-limited case with multiple detectors, i.e. a linear photodiode array detector. The model shows that the detection limit improves with the square root of the spectral bandpass and with the square root of the pixel width. A high resolution echelle monochromator and a low resolution (200 mm focal length) monochromator are both accurately described by the model. The two monochromators have comparable detection limits when the limiting noises are the same.

Graphite furnace atomic absorption spectrometry using a linear photodiode array and a continuum source

A graphite furnace atomic absorption spectrometer is described based on a continuum source and a linear photodiode array detector (LPDA). Wavelength integrated absorbance is computed which asymptotically approaches a maximum as more pixels are used to cover the absorption profile. The read noise of the LPDA was 3800 electrons. Absorbance noise computed from the propagation of errors and LPDA read noise accurately predicts the experimental results. Absorbance noise decreases as the entrance slit is increased and increases as the number of pixels in the calculation is increased. The best detection limits were found with an entrance slit-width of 500 µm and 16–32 pixels. The detection limits for Cd, Cu and Ni were comparable to line source atomic absorption spectrometry. Gated detection was necessary to eliminate source flicker when large numbers of pixels were used and in the presence of background absorbance.

Evaluation of linear photodiode array detection for continuum source atomic absorption spectrometry with electrothermal atomization

A 256 pixel linear photodiode array (LPDA) detector was fitted to a Spectrametrics echelle spectrometer for measurements by continuum source atomic absorption spectrometry with electrothermal atomization (ET-CSAAS). By selection of appropriate pixel regions, intensity values could be obtained for calculation of absorbance signals integrated over wavelength as well as time. Detection lmits obtained sequentially for Cd, Cr, Cu, Mn, Mo and Pb, with the LPDA, were similar in magnitude (4–24 pg) to those obtained with mechanical wavelength modulation and detection by a photomultiplier tube, using the same atomizer and spectrometer system. The calibration graphs obtained with the LPDA, however, were more useful analytically, as integration of the absorbance signals across the line profile increased the linear range. Four National Bureau of Standards (NBS)[now National Institute of Standards and Technology (NIST)] Standard Reference Materials, (SRMs) 1577a Bovine Liver, 1575 Pine Needles, 1572 Citrus Leaves and 1566 Oyster Tissue were analysed by ET-CSAAS with LPDA detection. The concentrations obtained for Cd, Cr, Cu, Mn, Mo and Pb were generally in good agreement with the certified values.

Simple Method to Measure the Coherence Time of a Mode-Locked Laser

A simple method is described to measure the coherence time of a mode-locked pulse train. The temporal coherence function is obtained from an interference pattern produced by crossing the two laser pulses at a linear photodiode array (PDA) detector. Two wave-fronts generated from the same or from successive pulses are tilted with respect to each other and intersect the PDA at a small angle; the “visibility” of the resulting fringe pattern is then plotted as a function of the temporal offset between the interfering pulses. The coherence time of a mode-locked Ar+ laser (225 ps) and that of a synchronously pumped mode-locked dye laser (5.0 ps) have been measured.

Application of a backside-illuminated charge-coupled-device camera for single-pulse coherent anti-Stokes Raman spectroscopy N_2 thermometry

The application of an unintensified backside-illuminated CCD for the acquisition of broadband single-pulse coherent anti-Stokes Raman spectroscopy (CARS) spectra is demonstrated. This CCD shows a quantum efficiency 5 times higher than a front-illuminated CCD and offers significant advantages compared with intensified linear photodiode array detectors generally used for single-pulse CARS thermometry. It overcomes the main drawbacks of the intensified linear photodiode array detector in single-pulse CARS N(2) spectroscopy: nonlinearity, limited dynamic range, and image persistence. A method for extending the dynamic range is demonstrated in a highly turbulent flame.

Observations of pattern evolution in thermal convection with high-resolution quantitative schlieren imaging

High-resolution measurements of the mean temperature gradient field of a fluid undergoing thermal convection in a vertical slot were made. The detection method employs schlieren optics with a linear photodiode array detector. With proper calibration the schlieren signals permit quantitative reconstruction of the varying temperature field along one dimension in the fluid, with a relative temperature resolution of ±0.02 K. The critical applied temperature difference, ΔTb,c, is determined by extrapolation of the measured convection pattern amplitudes. The measurements permit detailed monitoring of the wave number distribution and its development in time as well as harmonic analysis of the temperature waveform. The stability limits of convective patterns are investigated as a function of forcing and wave number and the observed limits are compared to theoretical predictions. The details of the transitions from conduction to convection and from one convective spatial pattern to another are observed and analyzed as the degree of forcing is varied. The creation and destruction of convective roll cells is observed as is the propagation of convective flow across the slot.

A Multi-Mode Spectrometer for Atomic Emission Spectrometry

A unique spectrometer system, the Multi-Mode Spectrometer (MMS), has been developed. The MMS integrates a scanning Michelson interferometer, a flat-field grating, and a linear photodiode array detector into a single spectrometer system. With these components, the MMS is capable of applying dispersive, interferometric, or combined dispersive/interferometric techniques for enhanced spectrometric flexibility. The effects of source fluctuation and redistributed photon noise can be reduced. In addition, the MMS has unique capabilities in data compression, application of internal standards, and noise spectrum analysis.

Evaluation of a Linear Self-Scanned Photodiode Array Detector for Direct Detection of 6-8 Kev X-Rays

ABSTRACT The performance of a commercially available 1-dimensional x-ray detector based on a directly bombarded self-scanned photo-diode array has been evaluated for x-rays in the 6-8 KeV range. The detector consists of a linear array of 1024 pixels, each approximately 25 μm wide by 2000 μm tall. Modifications to allow cooling to -55°C, which is required for long x-ray exposures, are described. The signal yield is 0.63 ADC counts/Fe55 x-ray, resulting in a detective quantum efficiency greater than 0.5 for doses over a range of 10 to 104 x-rays per pixel per scan when operated at -55°C. The resolution is about 3 pixels (75 μm FWHM) and the response with respect to dose is linear up to 1.5 x 104 x-rays integrated per pixel per scan. There is some non-linearity at doses below about 1000 x-rays/pixel/scan. The detector has very little geometric distortion. The uncorrected uniformity of response of the individual pixels (field flatness) is good to about 2%. However, the variation in responsiveness is stable and a field flatness calibration can be readily used to correct the uniformity of response.